The present invention relates to a liquid scintillation spectrometry process and apparatus permitting the analysis of samples having a quenching of the fluorescence efficiency resulting from both a colour quenching and a chemical quenching.
It is known that one of the disadvantages of liquid scintillation spectrometry is the quenching of the fluorescence efficiency (number of fluorescence photons emitted per unit of energy dissipated in the scintillator) which generally leads to a reduction in the measuring efficiency (number of pulses counted per unit of time for a given sample). This reduction cannot be calculated because it is dependent on a large number of parameters, many of which are not known at the time of measuring. It is therefore necessary to determine it experimentally for each sample.
A large number of processes have been suggested for this purpose. One for example consists of the use of an external standard radioactive source, whereby a supplementary measurement of the spectrum induced by it is performed in the sample. It is possible to determine the overall quenching of the sample by measuring the relationship of the counting rates in two different energy channels.
Another process is known which uses a standard external radioactive source and which no longer consists of merely measuring the quenching and instead also automatically corrects the sensitivity of the spectrometer in order to compensate this effect. The latter process formed the object of U.S. Pat. No. 3,560,744 granted to T. Jordan on Feb. 2, 1971 and entitled "Method and apparatus for compensating of quenching in liquid scintillation counting", the latter patent being considered as incorporated into the present description.
However, these correction methods are only accurate if the fluorescence quenching is not due both to chemical quenching and colour quenching. In this case the fluorescence efficiency of the solution is dependent on the respective contributions of these two effects in such a way that if said contributions are unknown, it is impossible to make a precise correction of the counting rate.
In order to facilitate the description which follows, it is pointed out that the term chemical quenching designates a phenomenon which leads to a reduction in the number of photons emitted by a liquid scintillator under the action of chemical agents, so-called quenchers, present in the solution. It is also pointed out that colour or chromatic quenching is a phenomenon which leads to a reduction in the number of photons reaching the detection means due to their partial absorption on passing through the liquid scintillator.
Thus, chemical quenching is due to a molecular process which occurs at the point of fluorescent radiation emission, whilst colour quenching occurs after said emission.
The necessity of taking separate account of these two types of quenching has already been noted. In his article entitled "Chemical vs. Color Quenching in Automatic External Standard Calibration; Application of Empirical Observations in a Computer Program" appearing on pages 823 to 833 of the work entitled "Organic Scintillators and Liquid Scintillation Counting" edited by D. L. Horrocks and Chin-Tzu Teng, 1971, Academic Press, the author, J. F. Lang, proposes an empirical method based on the use of four calibration equations established on the basis of a number of standard samples. These equations then make it possible to determine the true counting efficiency for each sample having colour and chemical quenchings in unknown proportions.
This question was again dealt with in the article by P. E. Stanley entitled "Fundamental Approaches for the Assessment of Chemical and Color Quenching in Backgrounds and Samples" published in the work "Liquid Scintillation Counting", 3, 1974, pp. 65-75, edited by M. A. Crook and P. Johnson.
In order to reduce the error made in conventional counting processes, a so-called "least amplitudes" analytical method was proposed which is described in the article by C. Ediss et al entitled "Lesser Pulse Height Analysis in Liquid Scintillation Counting" edited by P. E. Stanley and B. A. Scoggins, Academic Press, 1974, pp. 91-101. According to this process, instead of summating the pulses of each of the two photomultipliers for each scintillation as in conventional processes, only the smallest of the two pulses is used for each scintillation. Experience has shown that the calibration curves obtained with an external gamma source are much closer to one another for the two quenching types than in the case when pulse summation is used.
However, even with the latter process a difference exists and in addition pulse height statistics suffer from the reduced number of photoelectrons released at the cathodes of the photomultipliers, particularly in the case of scintillations caused by low energy beta-emitters such as, for example, H3.
Finally, reference can be made to the article by M. Takiue and H. Ishikawa entitled "Quenching Analysis of Liquid Scintillation" published in the Journal "Nuclear Instruments and Methods", 118, 1974, pp. 51-54 and this article is considered to be incorporated into the present description.